The technical part of the optician's profession, which consists in mounting a pair of ophthalmic lenses on a spectacle frame selected by a client, is divided into four main operations:                acquiring the shapes of the contours of the rims of the spectacle frame selected by the client,        centering each ophthalmic lens in a centering and blocking unit, this operation consisting in determining the frame of reference of the lens with the aid of centering marks provided on the latter, then in appropriately positioning the previously acquired contour of the rim in the frame of reference of the lens such that, once it has been cut out in a manner following this contour and then mounted in its frame, the lens is correctly positioned with respect to the corresponding eye of the client in order that it carries out to the best possible extent the optical function for which it has been designed,        blocking each lens, this operation consisting in fixing a blocking member to the lens such that the lens can be removed from the centering and blocking unit without losing the frame of reference, then        cutting out each lens, this operation consisting in machining this lens in a manner following the previously centered contour.        
During the centering and blocking operations, it is necessary to hold the lens in a fixed position.
For this purpose, a holder as mentioned above is known, the plate of which is transparent such that the lens can be centered with the aid of a conventional centering and blocking unit, and the studs of which together form a tripod on which the lens rests by way of its rear face.
This holder comprises in particular a metal base in the form of a three-point star which is fixed to the center of the transparent plate and which supports at each of its ends one of the three studs, such that the three studs are spaced in pairs 35 millimeters apart.
However, this lens holder does not prove to be entirely satisfactory both for centering the lens and for holding it in a fixed position.
Specifically, during the centering of the lens, the star-shaped base covers a part of the lens. It is thus possible for it to conceal the centering marks of the lens, with the risk that the centering and blocking unit can no longer center the lens.
Furthermore, while in the majority of cases the tripod holds the lens properly in a fixed position, sometimes it cannot correctly fulfill this function.
This is thus the case for example when the lens has already been cut out once and needs to be centered and machined a second time in order to perfectly fit the shape of the rim of the spectacle frame selected. Thus, specifically, sometimes the already cut lens has a smaller size which does not allow it to rest simultaneously on the three studs.
This is thus also the case when the lens has a very pronounced curvature and a large diameter, since there is a risk that its edge will come into contact with the transparent plate before its rear face rests on the tripod.
Furthermore, document FR2861005 discloses an ophthalmic-lens clamping jaw that can be used as a tripod for holding an ophthalmic lens.
This clamping jaw comprises three long arms mounted so as to be able to rotate on the holder about three parallel rotation axes.
The major drawback of this clamping jaw is that its arms, which are very long, cover a large part of the lens. It is thus possible for them to conceal the centering marks of the lens, with the risk that the centering and blocking unit cannot center the lens.